1. Field of the Invention
The present invention relates to a radiation image detecting device for detecting a radiation image.
2. Description Related to the Prior Art
In a medical field, an X-ray imaging system using radiation such as X-rays is known. The X-ray imaging system includes an X-ray generating apparatus for generating X-rays and an X-ray imaging apparatus for acquiring an X-ray image of an object (i.e. patient) from the X-rays that have passed through the object. The X-ray generating apparatus includes an X-ray source for irradiating the X-rays to the object, a source controller for controlling the operation of the X-ray source, and an irradiation switch for inputting a command for actuating the X-ray source to the source controller. The X-ray imaging apparatus includes an X-ray image detecting device for detecting the X-ray image by converting the X-rays having passed through the object into an electrical signal, and a console for controlling the operation of the X-ray image detecting device and storing and displaying the X-ray image.
An X-ray imaging apparatus using an X-ray image detecting device for electronically detecting an X-ray image has been widely spread instead of an X-ray image recording device using an X-ray film or an imaging plate (IP) cassette. The X-ray image detecting device has a sensor panel that is also referred to as a flat panel detector (FPD). The sensor panel has an imaging area in which a plurality of pixels for accumulating signal charges corresponding to a dose of incident X-rays are arranged in a matrix. Each of the pixels has a photoelectric converter for generating electric charges and accumulating the generated electric charges, and a switching element such as a thin film transistor (TFT). The sensor panel reads out the signal charges accumulated in the photoelectric converter of each of the pixels through a signal line disposed for each column of the pixels in a signal processing circuit upon turning on of the switching element, and the signal charges are converted into a voltage signal in the signal processing circuit. Thereby, an X-ray image is electronically detected.
Further, in the X-ray imaging system, in order to suppress radiation exposure to the object and acquire an X-ray image with appropriate image quality, automatic exposure control (AEC) is performed in some cases. In the AEC, a dose detection sensor measures a dose of X-rays during the X-ray imaging (i.e. during X-ray irradiation), and the X-ray irradiation from an X-ray source is stopped when an integrated value of the dose (i.e. an accumulated dose) reaches a target dose. The dose of the X-rays irradiated from the X-ray source is determined by a tube current-time product (mAs value) which is the product of X-ray irradiation time (in units of seconds “s”) and tube current (in units of milliamperes “mA”) for defining the dose of the X-rays to be irradiated from the X-ray source per unit of time. Each of the imaging conditions such as the X-ray irradiation time and the tube current has a rough recommendation value depending on a body part to be imaged (chest, head, and the like), the sex, the age, and the like of the object. However, X-ray transmittance varies in accordance with the individual difference such as a body frame of the object, and therefore the AEC is performed to achieve more appropriate image quality.
An ion chamber or the like has been conventionally used as the dose detection sensor. However, recently, there has been proposed a technique that pixels of a sensor panel are subjected to simple modification so as to function as a dose detection sensor. In an X-ray image detecting device disclosed in United States Patent Application Publication No. 2011/0180717 (corresponding to Japanese Patent Laid-Open Publication No. 2011-174908), some of pixels (hereinafter referred to as detection pixels) are each connected without intermediation of a switching element to a radiation detection line, and a dose signal corresponding to the electric charges generated in each of the detection pixels flow into the radiation detection line irrespective of whether the switching element is turned on or turned off. Then, the dose signal is sampled in an AEC section to which the radiation detection line is connected, and an integrated value of the sampled dose signals is calculated. Based on the calculated integrated value, it is determined whether or not the accumulated dose has reached the target dose.
As shown in FIG. 12, according to United States Patent Application Publication No. 2011/0180717, among pixels 200 arranged in 4-by-4 matrix, for example, the pixel in the first row and the second column, the pixel in the second row and the first column, the pixel in the third row and the third column, and the pixel in the fourth row and the fourth column are specified as detection pixels 200b (shown by hatching). Thus, the detection pixels 200b are dispersedly arranged such that the detection pixel 200b is disposed one-by-one in each row and each column. Further, in order to increase a level of the dose signal for the purpose of improving the decision accuracy in the AEC, the two detection pixels 200b including the pixel 200b in the first row and the second column and the pixel 200b in the second row and the first column are connected to a first radiation detection line 201a, and the two detection pixels 200b including the pixel 200b in the third row and the third column and the pixel 200b in the fourth row and the fourth column are connected to a second radiation detection line 201b, respectively, such that the dose signals from the former two pixels 200b and the dose signals from the latter two pixels 200b are added up and inputted to an AEC section 202.
The AEC section 202 integrates the additional value of the dose signals from the former two pixels 200b and the dose signals from the latter two pixels 200b at each sampling of the dose signals, and determines whether or not the accumulated dose has reached the target dose based on the integrated value. Namely, the integrated value of the additional values of the dose signals from the former two pixels 200b and the dose signals from the latter two pixels 200b is calculated as the accumulated dose for blocks 203a and 203b each of which consists of two rows and two columns and is shown by a thick frame in FIG. 12.
According to United States Patent Application Publication No. 2011/0180717, since a block 203c on the right of the block 203a and a block 203d on the left of the block 203b do not include the detection pixels 200b, it is impossible to obtain data of an accumulated dose in each of the blocks 203c and 203d. Therefore, the determination accuracy of the AEC section 202 is also decreased.